A few light taps upon the pane made him turn to the window. It had begun to snow again. He watched sleepily the flakes, silver and dark, falling obliquely against the lamplight. The time had come for him to set out on his journey westward. Yes, the newspapers were right: snow was general all over Ireland. It was falling on every part of the dark central plain, on the treeless hills, falling softly on the Bog of Allen and, farther westward, softly falling into the dark mutinous Shannon waves. It was falling, too, upon every part of the lonely churchyard on the hill where Michael Furey lay buried. It lay thickly drifted on the crooked crosses and headstones, on the spears of the little gate, on the barren thorns. His soul swooned softly as he heard the snow falling faintly through the universe and faintly falling, like the descent of their last end, upon all the living and the dead.
Okay, first things first: take a deep breath. You've just read (or reread) the last lines of "The Dead," and they're some of the most famous lines in 20th-century literature. Why are they so famous? First of all, the literary skill that goes into them is just about unparalleled, and it gives us the purest gold of Joyce's ability to put together words like nobody else. Revel in them for a sec, and then we'll get down to the nitty gritty.
In terms of senses, this passage calls on our senses of sight, touch, and hearing to perceive the snow, which is, by the way, both real snow and a part of Gabriel's imagination. On the one hand, it's a passage of concrete detail: we start out learning exactly where Gabriel is, and then are told both the direction of his vision and exactly what he sees. We even know the source of light that allows him to see the snow in the first place.
Then things start getting a little abstract. A "journey westward" refers to a trip to the West of Ireland, but it also suggests a journey toward the setting sun, toward darkness and toward death. It's concrete when it refers to the day's weather, and when it reports the places where snow is falling, but it's abstract at the same time as we realize that these aren't just places on a map, but places that are very, very significant for Gabriel.
When he thinks of the snow on Michael Furey's grave, we realize pretty suddenly that we aren't just looking at a weather map anymore—we're thinking, along with Gabriel, of a moment in the present that calls back to the past. And in this passage we're the proud perceivers of the effects of a literary device called anaphora: that word "falling" and then that word "lay" keep getting repeated, creating a lulling rhythm that mimics Gabriel's own nodding off to sleep.
Repetition is a big part of this passage in general. Did you notice how the word softly, which first describes the way that the snow falls, has an encore in the last sentence when it describes the way that Gabriel's soul "swooned"? It's almost unnoticeable, but that's the point: the way that snow falls is also almost unnoticeable, unless you're having the biggest epiphany of your entire life. Like Gabriel in the final sentence.
And you're right—it's a complex sentence from the start. The word "as" signals to us that there's something happening simultaneously, and we're supposed to be following both of those things. On the one hand, the movement of Gabriel's soul (the movement of souls is called metempsychosis for you vocab nerds out there), and on the other, the movement of the snow.
But wait, are those actually two things, or just one thing? Since his soul is swooning just as softly as the snow is falling, isn't his soul a lot like snow? That's where Joyce's next literary device comes in. He's using chiasmus, which is what we call that reversed order that makes "falling faintly" and "faintly falling" sound so totally awesome. And as if that wasn't enough, we get the wildest metaphor we've seen in Dubliners, which compares the snow's falling to "the descent of their last end." It's especially hard to interpret when we're reading because, the antecedent of "their" actually follows the pronoun. Phew.
This isn't usually fair, but Joyce is pulling out all the stops here and telling us that he needs to use this pronoun right here, right now because he's about to drop an amazing, amazing phrase and he needs it to come at just the right time. So as we're reading we're asking ourselves, "wait, whose last end?" and the suddenly we find out the answer to that as well as the conclusion of the simile.
Where was the snow falling, and whom was it falling on? Oh, "upon all the living and the dead." But even answering that basic question doesn't quite make it clear what he's getting at until we remember that "the descent of their last end" was a phrase used previously by Mary Jane to describe the way that monks sleep in their coffins. Now the phrase doesn't just describe a group of eccentric monks: it describes "all the living and the dead." That's the way Gabriel sees and hears this snow falling—as if it were death itself dropping all our swooning souls into little coffins all over the memory-dotted landscape of the country he's sick to death of. There's some paralysis for you.
One minute you were alive and giving a speech at a holiday party, and hoping to have some fun with your wife on the way home. The next you see a vision that's about as close to apocalyptic as it gets: everyone, even the living, are basically dead, so still that you can hear the snow—or your own soul—"falling through the universe." And that's what's up with the ending.